The role of hydrogen in the edge dislocation mobility and grain boundary-dislocation interaction in α-Fe

The atomistic mechanisms of dislocation mobility depending on the presence of hydrogen were investigated for two edge dislocation systems that are active in the plasticity of α-Fe, specifically ½<111>{110} and ½<111>{112}. In particular, the glide of the dislocation pile-ups through a single crystal, as well as transmission of the pile-ups across the grain boundary were evaluated in bcc iron crystals that contain hydrogen concentrations in different amounts. Additionally, the uniaxial tensile response under a constant strain rate was analyzed for the aforementioned structures. The results reveal that the presence of hydrogen decreases the velocity of the dislocations – in contrast to the commonly invoked HELP (Hydrogen-enhanced localized plasticity) mechanism -, although some localization was observed near the grain boundary where dislocations were pinned by elastic stress fields. In the presence of pre-exisiting dislocations, hydrogen-induced hardening was observed as a consequence of the restriction of the dislocation mobility under uniaxial tension. Furthermore, it was observed that hydrogen accumulation in the grain boundary suppresses the formation of new grains that leads to a hardening response in the stress-strain behaviour which can initiate brittle fracture points.     

Solute hydrogen effects on plastic deformation mechanisms of α-Fe with twist grain boundary

The deformation mechanisms in the α-Fe twist bi-crystals (TBCs) containing differently angled twist grain boundaries (TGBs) are investigated carefully using the molecular dynamics modeling, with especial concerns on how solute hydrogen affects them. The results show that there are three main deformations in the TBCs, i.e. the dislocation glide-dominated mechanism, the twining-dominated mechanism, the dislocation glide and twining co-dominated mechanism, depending upon both the twist angle and the loading direction. In the dislocation glide-dominated TBCs, solute hydrogen increases the dislocation nucleation strength, dislocation mobility and dislocation density, further increases the vacancies concentration due to frequent interactions of solute hydrogen atoms with dislocations. In the dislocation glide and twining co-dominated TBCs, the solute hydrogen has weaker effect on the increase of dislocations density and the decrease of twins fraction with increasing tensile strain. However, in the twining-dominated TBCs, solute hydrogen assists the deformation twinning but doesn't increase significantly the vacancies concentration. So, it seems that twinning deformation is beneficial to resist hydrogen embrittlement (HE). These knowledge is helpful for us to understand the HE mechanism and develop new hydrogen-resistant high-strength materials.     

Effect of boron on the hydrogen-induced grain boundary embrittlement in α-Fe

The influence of interstitial impurities such as B and C on the H-induced Fe Σ5(310) symmetrical tilt grain boundary embrittlement was investigated using the projector augmented-wave method. It was shown that in contrast to hydrogen, both boron and carbon decrease the grain boundary energy more significantly than the surface one. This results in an increase in the Griffith work, i.e. the grain boundary strengthening. The strengthening of grain boundary is more pronounced with increased number of B atoms whereas the increase of H concentration makes the process of intergranular brittle cleavage fracture easier. The grain boundary energy is lowered with an increased number of B atoms indicating a strong driving force for segregation. Our estimations of the Griffith work for the Fe Σ5(310) grain boundary containing both B and H atoms show an increase in comparison with the undoped grain boundary. It is revealed that improved cohesion of Fe Σ5(310) grain boundary due to B is mainly a chemical effect, whereas both elastic and chemical contributions to the Griffith work in case of H are negative, i.e. they are embrittling contributions.     

Effect of hydrogen on the creep of the ultrafine-grained zirconium Zr–1Nb alloy at 673 K

Comparative studies of the effect of hydrogenation to concentrations of 0.33 wt% on the behavior of the creep of Zr–1wt% Nb (hereinafter Zr–1Nb) alloy in fine-grained (d_av = 4 μm) and ultrafine-grained (d_av = 0.45 μm) states are performed for the creep rates in the interval 10^?7–10^?5 s^?1 at a temperature of 673 K. Hydrogenation to concentration of 0.33 wt% is found to lead to a decrease in the steady creep rate and increase in the time to failure with simultaneous decrease in the value of deformation to failure. Possible mechanisms of deformation of the ultrafine-grained alloy under examined creep conditions are discussed.     

Fundamental influence of hydrogen on various properties of α-titanium

First-principles calculation reveals that the Ti–H interactions are energetically favorable with negative heats of formation, and H atoms could occupy octahedral and tetrahedral interstitial sites of α-Ti simultaneously due to their small energy difference. Calculation also shows that hydrogen concentration plays an important role in determining brittle/ductile behavior of Ti–H phases, and densities of states suggest that a bonding transition from mainly covalent to mainly metallic appear for Ti–H phases when the H/Ti ratio reaches about 1/8. The calculated results agree well with experimental observations and could clarify the controversies of the Ti–H system in the literature.     

Process simulations of HI decomposition via reactive distillation in the sulfur–iodine cycle for hydrogen manufacture

Process simulations of HI decomposition via reactive distillation in the Sulfur–Iodine (S–I) cycle have been performed using heat pumps for energy recovery and a recently developed thermodynamic properties model. Several differences from previous flow sheets have been found through manual optimization of reflux ratio, number of stripping and rectifying stages, and pressure of the distillation column for typical inlet conditions to the HIx Section III. In particular, the RD column should have a minimal stripping section, can have as few as 10 total stages, an operating pressure of 12 bar, and a reflux ratio of 0.75, while achieving the production requirements. Though this design has limited improvement in energy requirements because the General Atomics energy recovery system is extremely effective, these results mean there should be a significant reduction in capital costs from prior estimates. In addition, as the inlet flow rate is increased, the input energy requirements decrease because of an increased ratio of H₂O to I₂ in the reboiler, lowering its temperature, and reducing the temperature differences for heat pump operations. The optimal inlet flow is between 126 and 140 mol/mol H₂, with a Section energy requirement of 367 kJ/mol H₂, and an overall process thermal efficiency estimated to be 41.5% relative to the higher heating value of hydrogen. These findings suggest there may be greater flexibility in conditions for the Bunsen reaction section as well as other possibilities for further energy efficiency improvement.     

Combustion behaviors of a direct-injection engine operating on various fractions of natural gas–hydrogen blends

Combustion behaviors of a direct injection engine operating on various fractions of natural gas–hydrogen blends were investigated. The results showed that the brake effective thermal efficiency increased with the increase of hydrogen fraction at low and medium engine loads and high thermal efficiency was maintained at the high engine load. The phase of the heat release curve advanced with the increase of hydrogen fraction in the blends. The rapid combustion duration decreased and the heat release rate increased with the increase of hydrogen fraction in the blends. This phenomenon was more obviously at the low engine speed, suggesting that the effect of hydrogen addition on the enhancement of burning velocity plays more important role at relatively low cylinder air motion. The maximum mean gas temperature and the maximum rate of pressure rise increased remarkably when the hydrogen volumetric fraction exceeds 20% as the burning velocity increases exponentially with the increase of hydrogen fraction in fuel blends. Exhaust HC and _CO2${\mathrm{CO}}_2$ concentrations decreased with the increase of the hydrogen fraction in fuel blends. Exhaust _NOx${\mathrm{NO}}_x$ concentration increased with the increase of hydrogen fraction at high engine load. The study suggested that the optimum hydrogen volumetric fraction in natural gas–hydrogen blends is around 20% to get the compromise in both engine performance and emissions.     

Molecular dynamics simulation on the effect of water uptake on hydrogen bond network for OH− conduction in imidazolium-g-PPO membrane

OH⁻ conduction involved in the hydrophilic channel of anion exchange membrane strongly depends on the water uptake. To investigate the effect of water uptake on the hydrogen bond network for OH⁻ conduction, a series of molecular dynamics simulations based on all-atom force field were performed on the hydrated imidazolium-g-PPO membranes with different water uptakes. The systems were well verified by comparing the membrane density and OH⁻ conductivity with previous experiments. By means of local structural properties and pair-potential energy, reasonable hydrogen bond criteria were determined to describe the hydrogen bond network confined in the membrane. Increasing water uptake enhances the hydration structures of water and OH⁻, and facilitates the reorganization of the hydrogen bond network. Water and OH⁻ are nearly saturated with water when the water uptake reaches λ = 10, where well-connected hydrogen bond network is produced. Further increasing water uptake has much less contribution to improving the hydrogen bond network, but inevitably swells the membrane channel. This work provides a molecular-level insight into the effect of water uptake on the hydrogen bonding structures and dynamics of OH⁻ and water confined in the imidazolium-g-PPO membrane.     

Catalytic activity of Co–Ag nanoalloys to dissociate molecular hydrogen. New insights on the chemical environment

Adsorption of molecular hydrogen on the surface of catalytic metal nanoparticles and its dissociation in atomic hydrogen are processes of interest in many chemical technologies. As in other chemical reactions, alloying can improve the efficiency of the catalysts. By focusing on Co₆, Co₅Ag, Co₃Ag₃ and CoAg₅, we explore the effect of changing the relative concentration of the two components in small Co_mAg_n clusters, a peculiar nanoalloy because Co and Ag do not form bulk solid alloys. Molecular hydrogen adsorbs preferentially on the Co atoms, and the binding is mainly due to the electrical polarization of the charges of adsorbate and host. The preference for Co sites and the trend in the strength of the H₂-cluster binding are explained by the combination of two effects characterizing the host environment. One of these is geometric, arising from the degree of exposure of the host atom: the lower the atomic coordination of the host atom, the stronger its bonding with H₂. The second effect, newly identified, reveals the importance of the chemical nature of the host atom environment: host Co atoms having both Co and Ag neighbors maintain their capacity to bind hydrogen more intact than those with only Co neighbors. The alloy nanoclusters catalyze the dissociation of adsorbed H₂ by building up quite small activation barriers. After dissociation, the H atoms occupy bridge positions between Co atoms (between Co and Ag in CoAg₅). H₂ adsorption and dissociation may trigger structural transformations of the cluster. The work shows that the adsorption and dissociation properties of H₂ can be tuned by varying the relative composition of the two atomic species in the nanoalloy.     

Effect of carbon addition on hydrogen storage behaviors of Li–Mg–B–H system

Various carbon additives were mechanically milled with LiBH₄/MgH₂ composite and their hydrogen storage behaviors were investigated. It was found that most of the carbon additives exhibited prominent effect on the host material. Among the various carbon additives, purified single-walled carbon nanotubes (SWNTs) exhibited the most prominent effect on the kinetic improvement and cyclic stability of Li–Mg–B–H system. Results show that LiBH₄/MgH₂ composite milled with 10 wt.% purified SWNTs additive can release nearly 10 wt.% hydrogen within 20 min at 450 °C, which is about two times faster than that of the neat LiBH₄/MgH₂ sample. On the basis of hydrogen storage behavior and structure/phase investigations, the possible mechanism involved in the property improvement upon carbon additives was discussed.     

Combustion of hydrogen–oxygen microfoam on the water base

On the basis of experimental study the paper analyzes the process of combustion wave propagation in the hydrogen–oxygen microfoam on the water base. Combustible microfoam consists of gaseous bubbles dispersed in the water solution of surfactant. Bubbles contain hydrogen and oxygen and their diameters are in the range from 60 to 230 μm. Expansion ratio of the combustible foam is in the range from 8 to 22. The paper establishes the influence of surfactant concentration, glycerol concentration, tube diameter and Shchelkin's spiral on the speed of flame propagation in the foam. It is shown that for considered range of regime parameters the characteristic mode of flame propagation in the semi-opened tube is the accelerated mode. The increase in glycerol content leads to the increase in flame speed. However after certain critical concentration of glycerol the foam loses the ability to burn. Total burning rate depends on surfactant concentration non-monotonically with characteristic maximum. Shchelkin's spiral installed at inner surface of the tube as well as the decrease in tube diameter favor flame deceleration.     

Cobalt doping of α-Fe/Al2O3 catalysts for the production of hydrogen and high-quality carbon nanotubes by thermal decomposition of methane

Fe–Co/Al₂O₃ catalysts were developed and tested in the catalytic decomposition of methane (CDM) for the synthesis of multi-wall carbon nanotubes (MWCNT) and the CO₂-free hydrogen production. While Fe (54.5–66.7 mol.%) is the main active phase for the carbon formation on the catalyst, Co acts as dopant aiming to improve its overall catalytic behaviour. Catalysts with Co contents of up to 18.2 M% showed the presence of α-Fe and Fe–Co crystallites with different size and lattice parameter. Fe_1-xCo_x alloy with bcc crystal system was identified only for Co contents of 14.0% and above, and presented a lattice constant lower than α-Fe, which would modify the carbon diffusion of the metal particle during the MWCNT growth. Co inhibited the Fe₃C formation during CDM resulting in higher carbon formations and longer activity times. This phase, shown in undoped catalysts, favored the presence of bamboo-type carbon nanotubes.     

The role of induced α′-martensite on the hydrogen-assisted fatigue crack growth of austenitic stainless steels

The effects of rolling on the hydrogen-assisted fatigue crack growth characteristics of AISI 301, 304L and 310S stainless steels (SSs) were investigated. In hydrogen, cold rolled specimens with a 20% thickness reduction were found to increase the fatigue crack growth rates (FCGRs) in the 301 and 304L SSs, and to a much lesser extent in the 310S SS. However, enhanced slip was observed for the 310S specimen in hydrogen. Hydrogen-accelerated FCGRs of the 301 and 304L SSs were related with the crack growth through the strain-induced martensite formed in the plastic zone ahead of the crack tip.     

Investigation of the role of Nb on Pd−Zr−Zn catalyst in methanol steam reforming for hydrogen production

Methanol steam reforming is regarded as a very promising process to generate H₂ suitable for fuel cells. Typically, the Pd-based catalysts can catalyze efficiently methanol steam reforming for hydrogen production. But their high selectivity to CO, a byproduct of methanol reforming reaction, severely limits their potential application. In this work, a series of Nb-modified Pd−Zr−Zn catalysts with different Nb loadings were prepared to study their catalytic activities with more focus on the role of Nb on Pd−Zr−Zn catalyst for methanol steam reforming. The prepared catalysts were fully analyzed by using various characterization techniques, for example, ICP, BET, SEM, XRD, H₂-TPR, NH₃-TPD, HRTEM, CO chemisorption, XPS, and Raman. The experimental results showed that an increase in Nb loading for the Nb-modified Pd−Zr−Zn catalysts led to a decrease of the methanol conversion and H₂ production rate. This was probably due to the decrease in the amount of oxygen vacancies on the catalyst surface. However, introduction of Nb into Pd−Zr−Zn catalyst increased the acid strength on the catalytic surface. The aldehyde species derived from methanol decomposition were readily transformed to HCOOH, thus yielding high selectivity to CO₂ for the Nb-modified Pd−Zr−Zn catalysts. Significantly, the addition of Nb to Pd−Zr−Zn catalyst facilitated the incorporation of Pd into the ZnO lattices, which led to the formation of Pd−Zn alloy. Consequently, the Nb-modified Pd−Zr−Zn catalysts exhibited significantly lower CO selectivity and production rate than the Pd−Zr−Zn catalyst. From the results, this work offers a new way to the rational design of selective methanol steam reforming catalysts to decrease the formation of byproduct CO.     

Generalized model of thermal boundary conductance between SWNT and surrounding supercritical Lennard-Jones fluid – derivation from molecular dynamics simulations

Using classical molecular dynamics simulations, we have studied thermal boundary conductance (TBC) between a single-walled carbon nanotube (SWNT) and surrounding Lennard-Jones (LJ) fluids. With an aim to identify a general model that expresses the TBC for various surrounding materials, TBC was calculated for three different surrounding LJ fluids, hydrogen, nitrogen, and argon in supercritical phase. The results show that the TBC between an SWNT and surrounding LJ fluid is approximately proportional to local density (ρ_L) formed on the outer surface of SWNT and energy parameter (ε) of LJ potential, and inverse proportional to mass (m) of surrounding LJ fluid. In addition, the influence of the molecular mass of fluid on TBC is far more than other inter-molecular potential parameters in realistic range of molecular parameters. Through these parametric studies, we obtained a phenomenological model of the TBC between an SWNT and surrounding LJ fluid.     

A density functional theory and grand canonical Monte Carlo simulations study the hydrogen storage on the Li-decorated net-τ

To find ideal hydrogen storage media, hydrogen storage performance of Li decorated net-τ has been investigated by first-principles calculations. Maximum 6 Li atoms are adsorbed on net-τ, with the average binding energy of 2.15 eV for per Li atom. Based on 6Li-decorated net-τ, up to twenty H₂ molecules are adsorbed, with a high H₂ storage capacity of 12.52 wt% and an appropriate adsorption energy of 0.21 eV/H₂. Finally, H₂ uptake performance is measured by GCMC simulations. Our results suggest that Li-decorated net-τ may be a promising hydrogen storage medium under realistic conditions.     

Research on the design of hydrogen supply system of 70 MPa hydrogen storage cylinder for vehicles

A hydrogen supply system of 70 MPa hydrogen storage cylinder on vehicles is designed, in which a compressor is proposed to use the new type of ion compressor. The system is simulated statically by Aspen Plus. Meanwhile, during the process of hydrogen charged from the third-stage high-pressure hydrogen storage tank to the hydrogen storage cylinder on vehicles, the dynamic variety of the third-stage high-pressure hydrogen storage tank is simulated dynamically by Aspen HYSYS Through the simulation, obtaining the results that there are difference between theoretical calculation and simulation for the volume of third-stage high-pressure hydrogen storage tank and the average volume flow of hydrogen in a third-stage high-pressure hydrogen storage tank varies with its pressure and volume. By comparing the results of Aspen Plus simulation and Aspen HYSYS simulation, there are some differences. The designed system can be applied to hydrogen stations and any operating conditions involving the supply hydrogen.     

Adapting the French nuclear fleet to integrate variable renewable energies via the production of hydrogen: Towards massive production of low carbon hydrogen?

Producing low-carbon hydrogen at a competitive rate is becoming a new challenge with respect to efforts to reduce greenhouse gas emissions. We examine this issue in the French context, which is characterised by a high nuclear share and the target to increase variable renewables by 2050. The goal is to evaluate the extent to which excess nuclear power could contribute to producing low-carbon hydrogen.Our approach involves designing scenarios for nuclear and renewables, modelling and evaluating the potential nuclear hydrogen production volumes and costs, examining the latter through the scope of hydrogen market attractiveness and evaluating the potential of CO₂ mitigation.This article shows that as renewable shares increase, along with the hydrogen market expected growth driven by mobility uses, opportunities are created for the nuclear operator. If nuclear capacities are maintained, nuclear hydrogen production could correspond to the demand by 2030. If not, possibilities could still exist by 2050.     

Preparation and characterization of α-Fe2O3 supported clay as a novel photocatalyst for hydrogen evolution

Improvement in the hydrogen evolution is reported over α-Fe₂O₃ supported on Algerian natural clay. The hetero-system is prepared by impregnation and calcination at 450 °C. It was characterized by X-ray diffraction, SEM analysis, FTIR spectroscopy and photo electrochemistry. The hematite Fe₂O₃ crystallizes in the corundum structure and exhibits n-type conductivity with a flat band potential of −0.88 V_SCE. Hence, the photo electrons located in Fe₂O₃-CB (−1 V_SCE) have high ability to reduce water into hydrogen. α-Fe₂O₃ gets effectively dispersed in the clay and the photoactivity increases with increasing its content. SO₃²⁻, working as hole scavenger, provides an absolute protection against the photo corrosion and favors the charges separation. The best performance of H₂ evolution occurs at alkaline pH on 10% Fe₂O₃/clay with a liberation rate 0.121 μmol/mg/min and a quantum efficiency of 1.2%.     

The effect of hydrogen injection parameters on the quality of hydrogen–air mixture formation for a PFI hydrogen internal combustion engine

To research the quality of the hydrogen–air mixture formation and the combustion characteristics of the hydrogen fueled engine under different hydrogen injection timings, nozzle hole positions and nozzle hole diameter, a three-dimensional simulation model for a PFI hydrogen internal combustion engine with the inlet, outlet, valves and cylinder was established using AVL Fire software. In the maximum torque condition, research focused on the variation law of the total hydrogen mass in the cylinder and inlet and the space distribution characteristics and variation law of velocity field, concentration field and turbulent kinetic energy under different hydrogen injection parameters (injection timings, nozzle hole positions and nozzle hole area) in order to reveal the influence of these parameters on hydrogen–air mixture formation process. Then the formation quality of hydrogen–air mixture was comprehensively evaluated according to the mixture uniformity coefficient, the remnant hydrogen percentage in the inlet and restraining abnormal combustion (such as preignition and backfire). The results showed that the three hydrogen injection parameters have important influence on the forming quality of hydrogen–air mixture and combustion state. The reasonable choice of the nozzle hole position of hydrogen, nozzle hole diameter and the hydrogen injection time can improve the uniformity of the hydrogen–air mixing in the cylinder of the hydrogen internal combustion engine, and the combustion heat release reaction is more reasonable. At the end of the compression stroke, the equivalence ratio uniform coefficient increased at first and then decreased with the beginning of the hydrogen injection. When hydrogen injection starting point was with 410–430°CA, equivalence ratio uniform coefficient was larger, and ignition delay period was shorter so that the combustion performance index was also good. And remnant hydrogen percentage in the inlet was less, high concentration of mixed gas in the vicinity of the inlet valve also gathered less, thus suppressing the preignition and backfire. With the increase of the distance between the nozzle and the inlet valve, the selection of the hydrogen injection period is narrowed, and the optimum hydrogen injection time was also ahead of time. The results also showed that it was favorable for the formation of uniform mixing gas when the nozzle hole diameter was 4 mm.